The present invention relates to the measurement of steam quality. In particular, the invention relates to a method and apparatus for determining steam quality which involves adding heat to or removing heat from a sample of steam to obtain a divergence in pressure or temperature that occurs when the sample is superheated or subcooled.
The quality of a two-phase flow is a parameter of importance in the operation and design of heat exchangers, turbines, steam engines, pumps, piping systems and the like. In heat exchangers that operate at saturated conditions such as steam generators and condensers, performance is characterized by the state of fluid entering and leaving the heat exchanger. In order to determine the state of the fluid at saturation conditions, the inlet or exit quality must be known. In steam generators, for example, lower than expected outlet quality is an indication of poor heat transfer performance of the steam separation equipment. In turbines, high moisture content at the turbine inlet can cause turbine blade erosion as well as reduced steam efficiency.
Several techniques have been used to measure the quality of steam. One such technique is disclosed in U.S. Pat. No. 4,034,597 to Fredriksson in which a sample of steam is introduced into a variable-volume sampling chamber that is kept at constant temperature. The chamber is placed in the path of the steam flow to be sampled. After a sample of steam is introduced into the chamber, the chamber is closed and maintained at a constant temperature by allowing steam to flow past the chamber. The volume of the chamber is then increased and pressure is monitored to sense a drop in pressure that occurs once the entire liquid portion of the sample has evaporated.
U.S. Pat. No. 4,833,688 to Smith discloses a device for measuring steam quality that incorporates an evacuated sample chamber into which steam is drawn until the pressure within the chamber equals that within the steam flow. The device operates on the principal that a two-phase mixture, when throttled into a closed tank, will become superheated. The internal energy and enthalpy of the superheated mixture can be determined based on pressure and temperature measurements, which then allows the quality of the mixture to be computed. Before the steam sample is taken, the sample tank is preferably maintained at the same temperature as the source steam by using a heating element. Once the sample is taken, the tank is not heated by an external source.
U.S. Pat. No. 4,149,403 to Muldary et al discloses a device for measuring steam quality that incorporates sensors for measuring pressure in an unrestricted portion of the steam flow in comparison to pressure in a restricted, critical flow portion. The enthalpy of the steam is determined from experimentally generated curves which then allows for the calculation of steam quality.
U.S. Pat. No. 4,574,626 to Kaya et al discloses a method for measuring the enthalpy of a two-phase substance. Temperature, pressure and density measurements of the substance are made and tables are used to calculate the enthalpy of the substance.
U.S. Pat. No. 4,969,084 to Smith discloses a method for calculating the enthalpy of steam based on pressure, temperature and flow measurements.
U.S. Pat. No. 4,891,948 to Kure-Jensen et al discloses a steam turbine thermal performance monitor in which enthalpy is calculated based on various temperature and pressure measurements.
U.S. Pat. No. 4,471,620 to Binstock et al discloses a turbine low pressure bypass spray valve control system in which enthalpy is calculated based on temperature, pressure and flow measurements. The amount of cooling water necessary for injection into the steam flow is then determined.
U.S. Pat. No. 4,827,429 to Silvestri, Jr. discloses a method for determining turbine impulse chamber temperature which includes the calculation of enthalpy and entropy values based on various steam pressure and temperature measurements.
U.S. Pat. No. 4,776,301 to Dziubakowski discloses a method of controlling steam temperature that involves the measurement of flow, temperature and pressure to maintain a constant enthalpy of steam entering and leaving a superheater.
U.S. Pat. No. 4,402,183 to Dimitroff et al. discloses a pressure flash tank in which steam pressure is maintained at a constant level with varying amounts of input water flow and enthalpy values. The system includes the use of pressure transducers at various locations.
The present invention has been developed in view of the foregoing and to overcome the deficiencies of the prior art.